In a wireless communication system, a base station communicates with several remote terminals such as mobile cell phones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are traditional multiple access schemes used to supply simultaneous services to a number of terminals. The basic idea in FDMA and TDMA systems is to respectively share the available resource at several frequencies or several time intervals so that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to FDMA and TDMA systems in the sense that transmission and reception takes place at different frequencies, and also at different time intervals. Unlike these systems using frequency division or time division, CDMA (code division multiple access) systems enable several users to share a common frequency and a common time channel by using a coded modulation. CDMA systems include the CDMA 2000 system, the WCDMA (Wide Band CDMA) system and the IS-95 standard.
A scrambling code is used in CDMA systems associated with each base station to distinguish one base station from another, as readily understood by one skilled in the art. Furthermore, an orthogonal code known as the OVSF code is allocated to each remote terminal such as a mobile cell phone, for example. All OVSF codes are orthogonal with each other so that one remote terminal can be distinguished from another.
Before a signal can be sent on the transmission channel to a remote terminal, the signal is scrambled and spread by the base station using the scrambling code for the base station and the OVSF code for the remote terminal. Consequently, the signal symbols are transformed into chips. The chip rate is higher than the symbol rate.
DS-CDMA systems refer to CDMA systems using spread spectrum signals. Conventionally, this type of receiver includes a radio frequency analog stage connected to an antenna to receive a spread spectrum signal.
The radio frequency stage comprises a low noise amplifier and two processing channels comprising mixers, filters and conventional amplifiers. The two mixers receive two corresponding signals at a phase difference of 90° from a phase locking loop comprising a local oscillator. After the frequency has been transposed in the mixers, the two processing channels define two paths I and Q, which are in quadrature. The signal is then transposed in the frequency base band of the signal.
After digital conversion in analog/digital converters, the I and Q paths are output to a reception processing stage that conventionally comprises a Rake receiver. The Rake receiver is typically used in mobile cell phones operating in a DS-CDMA communication system, and is used for time alignment, descrambling, despreading and a combination of delayed versions of the initial signals so as to output information contained in the initial signals.
The frequency transposition of the signal received in the base band usually leads to inaccuracies. This is mainly due to the quality of the local oscillator which then causes a residual frequency offset error. The digital processing stage of the receivers usually comprises an automatic frequency control (AFC) algorithm, the objective of which is to minimize the residual frequency error to bring it down to an acceptable value that has no influence on subsequent signal processing.
There are several available methods of frequency correction. Automatic frequency control techniques used in narrow band communications may be used for frequency control after despreading. Several methods are discussed in the following references: A. N. D'Andrea and U. Mengali, “Design Of Quadricorrelators For Automatic Frequency Control Systems”, IEEE Trans. Commun., vol. 41, pp. 988–997, January 1993; M. L. Fowler and J. A. Johnson, “Extending The Threshold And Frequency Range For Phase-Based Frequency Estimation”, IEEE Trans. Signal Processing, vol. 47, pp. 2857–2863, October 1999; and A. Wannasarnmaytha, S. Hara and N. Morinaga, “Two Step Kalman Filter Based AFC For Direct Conversation-Type Receiver In LEO Satellite Communications”, IEEE Trans. Commun., vol. 49, pp. 246–253, January 2000.
However, performances of these techniques do not satisfy a number of requirements of third generation receivers designed to operate under the CDMA standard. For example, a wide correction frequency range, fast correction and low implementation complexity are not satisfied. More precisely, correction ranges of automatic frequency control techniques used in a narrow band are insufficient when the frequency is high and the frequency offset is of the order of the symbol rate, as in third generation wireless communication systems for example.
Automatic frequency control techniques with dual filters and with balanced quadricorrelators as disclosed in the D'Andrea et al. article use correction ranges on the order of the symbol rate. However, these techniques involve the use of complicated filters to eliminate inductive noise and require precise knowledge about the transmission channel.
Other automatic frequency control techniques (Jing Lei and Tung-Sang Ng, “New AFC Algorithm For A Fully-Digital MDPSK DS/CDMA Receiver”, ISCAS 2001, vol. 4, pp. 294–297) also use correction ranges on the order of the symbol rate, but their implementation is very complex due to the fact that they must be applied before signal despreading.